1. Field of the Invention
The invention relates to the improvement of a duo-servo type drum brake device for use in rear brakes or transmission brake of automobiles.
2. Description of the Related Art
The brake devices of this kind are usually constructed as shown in FIG. 6, for example.
A pair of brake shoes 51, 52 are disposed opposite to each other so as to abut against a fixed anchor member 53 at one adjacent end portions thereof, which are located on the upper portion of the drawing, and so as to abut against both ends of a shoe gap adjuster 54 at the other adjacent end portions thereof.
Shoe return springs 55 to 57 are provided to cause both the brake shoes 51, 52 to abut against the anchor member 53 and the shoe gap adjuster 54.
A wheel cylinder 58 is disposed between both of the brake shoes 51, 52 adjacent to the anchor member 53. A parking brake operating lever 60 is pivoted on the end portion of the brake shoe 52 on the side of the anchor member 53 by means of a pin 59, and a strut 61 is disposed between the intermediate portion between the basal end and the operating side end of the parking brake operating lever 60, and the brake shoe 51.
According to the above construction, the supply of the braking hydraulic pressure to the wheel cylinder 58 by the service brake causes both the brake shoes 51, 52 to be opened in the direction away from each other around abutment points with the shoe gap adjuster 54, respectively, and then to be pressed to the inner circumferential surface of the brake drum 62. On this occasion, both the brake shoes 51, 52 are apart from the anchor member 53, respectively.
There will be described a case in which a braking action caused when the brake drum 62 receives a torque in the direction shown by the arrow R will now be described. The rotation of the brake drum 62 causes the brake shoes 51, 52, the shoe gap adjuster 54, and the parking brake operating lever 60 to be rotated in the direction shown by the arrow R, thereby causing the brake shoe 51 to be displaced in the corresponding direction according to the gap with the anchor member 53 and then to be struck thereagainst, with impossible rotation, which brings about a predetermined braking action.
When the brake drum 62 receives a torque in the direction opposite to that shown by the arrow R, a predetermined braking action is not caused until the brake shoe 52 is displaced in the above-mentioned opposite direction according to the gap with the anchor member 53 to be struck thereagainst.
When the parking brake applies the operating force shown by W in FIG. 6 to the operating end of the parking brake operating lever 60, the lever 60 is rotated around the pin 59 in the counterclockwise direction, thereby causing the brake shoe 51 to be opened through the strut 61.
This causes the brake shoe 51 to be contacted to the inner circumferential surface of the brake drum 62, thereby causing the lever 60 to be rotated around the abutment point with the strut 61 in the counter-clockwise direction as is the case with above case, which causes the brake shoe 52 to be opened through the pin 59.
As a result, both the brake shoes 51, 52 are friction-contacted to the inner circumferential surface of the brake drum 62, which provides a predetermined parking brake action on the same principle as the service brake.
Then the braking action due to the above-mentioned service brake and parking brake indispensably causes the brake shoe 51 or 52 to strike the anchor member 53, thereby inevitably bringing about a tap at the time of the shoe striking mentioned-above, which makes a driver or passengers feel uncomfortable.
Also, since the brake shoe 51 or 52 repeatedly strikes the anchor member 53, the anchor member 53 itself and the attaching portion of the anchor member 53 are required to be improved in strength; therefore the conventional duo-servo type drum brake device shows a tendency to be increased in weight and in cost, with disadvantageous results.
Further, when the brake drum 62 receives a torque in the direction shown by the arrow R in FIG. 6 during using the parking brake, the brake action is not caused until the brake shoes 51, 52, the shoe gap adjuster 54, and parking brake operating lever 60 are rotated in the direction shown by the arrow R according to the above-mentioned gap with the rotation of the brake drum 62, during which the parking brake cable undesirably loosens, which may cause the braking force to be lowered.